1. Field of the Invention
This invention relates to vertical seismic profiling and more particularly to a method for reducing noise in a detected signal.
2. Description of the Prior Art
Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface impendance contrasts. Presently a technique commonly known as Vertical Seismic Profiling (VSP) is utilized to image the subsurface in the vicinity of a borehole. With VSP, typically, a source is activated at the surface and sensors are positioned at discrete borehole depths with a wireline. More recently methods have been developed for performing VSP with a downhole source and with receivers positioned on the surface. With the source positioned downhole, data can be acquired simultaneously at many surface locations with little more expense than for a single location. One such method utilizes the drill bit as the downhole source. In this method, a sensor is positioned near the top of the drill string to record a reference or pilot signal and field sensors are positioned at selected locations in the vicinity of the wellbore. The signal generated by the drill bit travels up the drill string to the reference (pilot) sensor and also propagates from the drill bit upward to the field sensors. The drill bit signal also propagates downwardly to reflecting subsurface interfaces and back to the field sensors. By correlating the signal detected by the pilot sensor with the signal detected by the field sensors, the travel time of the energy traveling from the drill bit to the field sensors may be determined. The travel times may then be utilized to map the location of the reflecting subsurface interfaces. Noise which is present in the pilot signal will degrade the quality of the correlated signal. It is an object of this invention to reduce the noise in the signal detected by the pilot sensor.